In order to meet the requirements of future computing systems, higher speed and more energy efficient alternatives to electrical interconnects such as, for example, on-chip optical interconnects and chip-to-chip optical interconnects, may be needed. Integrated optics, particularly silicon photonics, may suitably meet such needs. For the cost-effective, mass-fabrication of complementary metal-oxide semiconductor (CMOS)-based chips having a performance capability suitable for use in high-speed devices and/or applications, integrated optical interconnects with compatible light sources are to be provided. A problem in this regard is that, due to the indirect band-gap of silicon, no silicon-based light sources are available and/or may be used. This problem has been addressed by the use of III-V based semiconductor material systems typically being used as light sources in conjunction with silicon photonics and, more generally, integrated optics based on a silicon platform. However, an associated problem in this regard is posed by the lattice mismatch between III-V compound semiconductors and silicon, making the direct, monolithic integration of III-V based light sources on a silicon platform non-trivial. In previously-proposed approaches for facilitating such integration, bonded III-V based, pre-processed light sources or blanket gain materials have been used. In this regard, it may be time-consuming and challenging to achieve relatively high-precision alignment when bonding a pre-processed III-V based light source to a given waveguide structure, particularly since the alignment precision may be further limited by the bonding process. For bonding a blanket III-V material on a pre-processed silicon-based waveguide, the alignment marks located on the silicon wafer that are provided for the lithography step involved in the patterning of the III-V layer may be used. Because the alignment accuracy of light sources based on compound semiconductor systems, such as, for example, III-V materials, with respect to optical structures, such as, for example, silicon waveguides and/or resonators, may be rather dependent on lithography accuracy, it may be insufficient for certain applications. Generally, the positioning accuracy of such heterogeneously/hybrid integrated optical systems is inherently lower compared to monolithic integration. Furthermore, concerning yield, heterogeneous/hybrid integration may not match the standards provided by integrated electronics. In this regard, and typically, yield features are in the 90% range for single devices, for example.
A further problem to be considered for heterogeneous/hybrid optical systems, particularly for III-V based light sources used with silicon photonics, is concerned with how the generated light is located in such systems. In this regard, it may be that the light is located mainly in the silicon with a relatively smaller overlap with the III-V material system, which may result in a relatively low material gain and high threshold currents. Or, it could be that the light is located mainly in the III-V material system in which case there is a relatively good overlap with the gain sections but also the possibility of lossy resonators that may contribute to relatively lower optical output power.
Previously-proposed devices/systems fabricated using hybrid/no integration on a silicon platform have been described in the following documents: “Electrically pumped hybrid AlGaInAs-silicon evanescent laser”, by Fang et al., published in Optics Express, vol. 14, issue 20, pp. 9203-9210, 2006; “Electrically pumped InP-based microdisk lasers integrated with a nanophotonic silicon-on-insulator waveguide circuit”, by Van Campenhout et al., published in Optics Express, vol. 15, issue 11, pp. 6744-6749, 2007; “Metamorphic DBR and tunnel-junction injection: A CW RT monolithic long-wavelength VCSEL”, by Boucart et al., published in IEEE J. Sel. Topics Quantum Electron, vol. 5, issue 3, pp. 520-529, 1999, and “Pseudomorphic and metamorphic quantum dot heterostructures for long-wavelength lasers on GaAs and Si”, by Mi et al. published in IEEE Journal of selected topics in quantum electronics, vol. 14, no. 4, pp. 1171-1179, 2008.
In the document titled, “III-V/Si photonics by die to wafer bonding”, by Roelkens et al. published in Materials Today, vol. 10, issues 7-8, pp. 36-43, 2007, the bonding of a III-V layer to a silicon wafer using a polymer adhesive is described.
This document does not seem to contain any teaching on optical properties and/or functionalities as per an optical device/system suitable for use in a given optical application that may address/provide a solution to the problems associated with previously-proposed optical devices/systems as above-described with reference to hybrid/heterogeneous integrated optical systems.
Reference is now made to the document titled, “III-V semiconductor-on-insulator n-channel metal-insulator-semiconductor field-effect transistors with buried Al2O3 layers and sulphur passivation: Reduction in carrier scattering at the bottom interface”, by Yokoyama et al. published in Appl. Phys. Lett., vol. 96, 142106, 2010, in which the fabrication of a III-V transistor on silicon by bonding a seed-layer and performing successive growth is described. This document does not seem to contain any teaching on optical properties and/or functionalities as per an optical device/system suitable for use in a given optical application that may address/provide a solution to the problems associated with previously-proposed optical devices/systems as above-described with reference to hybrid/heterogeneous integrated optical systems.
In the document titled, “Defect reduction of GaAs epitaxy on Si (001) using selective aspect ratio trapping”, by Li et al. published in Appl. Phys. Lett., vol. 91, 021114, 2007, in which III-V epitaxy in oxide trenches on silicon has been reported using aspect ratio trapping. Reference is also made to the document titled, “Monolithic integration of GaAs/InGaAs lasers on virtual Ge substrates via aspect-ratio trapping”, by Li et al. published in J. Electrochem. Soc. 156, H574, 2009, in which the formation of GaAs/InGaAs quantum well lasers, by metallorganic chemical vapour deposition, on virtual Ge substrates on silicon has been demonstrated via aspect ratio trapping and epitaxial lateral overgrowth. These documents are respectively concerned with addressing known problems associated with the fabrication of structures comprising compound semiconductor material systems, such as III-V material systems, on silicon, which may cause performance deterioration of devices in which such structures are integrated. Such problems are related to the lattice mismatch and difference in thermal coefficients between III/V material systems and silicon. However, neither of these documents address the problems, as discussed hereinabove, with respect to hybrid/heterogeneous integrated optical systems and/or the monolithic integration and optical coupling of light sources with optical structures such as waveguides and, more generally, photonic structures.
The epitaxial growth of III-V materials directly on a silicon platform and/or light-emitting devices and photodetectors based on nanowires have been reported in the following documents: “Si—InAs heterojunction Esaki tunnel diodes with high current densities”, by Bjoerk et al., published in Appl. Phys. Lett., vol. 97, 163501, 2010; “Nanolasers grown on silicon” by Chen et al., published in Nature Photonics, vol. 5, pp. 170-175, 2011, and “GaAs based nanoneedle light-emitting diode and avalanche photodiode monolithically integrated on a silicon substrate” by Chuang et al., published in Nano Letters, vol. 11, pp. 385-390, 2011. These documents do not seem to address the monolithic integration of III-V based light sources on a silicon platform. Furthermore, they do not address/propose any solution/alternative to the problems and/or issues associated with hybrid/heterogeneous integrated optical systems.
Reference is now made to the document titled, “Hybrid III-V semiconductor/silicon nanolaser”, by Halioua et al., published in Optics Express, vol. 19, 9221, 2011, in which an optically pumped one-dimensional photonic cavity laser is vertically coupled to a pre-structured straight silicon waveguide. Alignment of the laser with respect to the silicon waveguide is performed by electron-beam lithography using markers formed in the silicon waveguide, with an overlay accuracy of better than 50 nm potentially being achieved. This document does not disclose any electrical pumping and/or lateral coupling of the light source with the passive optical components/aspects. Furthermore, this document does not seem to provide instruction on how to address the problems/issues associated with hybrid/heterogeneous integrated optical systems as hereinbefore described.
In the document titled, “Design and optical characterisation of photonic crystal lasers with organic gain material”, published by Baumann et al. in Journal of Optics, vol. 12, 065003, 2010, spin-coating of an organic gain material onto a two-dimensional photonic crystal is reported. Whilst suitable for organic gain material, spin-coating is not compatible with respect to solid state gain materials, such as, for example, III-V material systems. Furthermore, this document does not disclose any electrical pumping.
Turning to the document titled, “Ultra-high quality-factor resonators with perfect azimuthal modal-symmetry”, published by Moll et al. in Optics Express, vol. 17, 20998, 2009, a circular grating and the use of the disclosed devices as modulators is described. No teaching is provided on electrical pumping or the integration of III-V material systems with silicon.
US 2008/0128713 A1 discloses a light-emitting device including a first electrode unit for injecting an electron, a second electrode unit for injecting a hole, and light-emitting units electrically connected to the first electrode unit and the second electrode unit, respectively, wherein the light-emitting units are formed of single-crystal silicon, the light-emitting units having a first surface (topside surface) and a second surface (underside surface) opposed to the first surface, plane orientation of the first and second surfaces being set to a (100) plane, thicknesses of the light-emitting units in a direction orthogonal to the first and second surfaces being made extremely thin. This document describes the fabrication and use of a silicon light source and a silicon laser. The process by way of which light is generated seems to be based on impact ionisation rather than on a direct band-gap transition. Furthermore, neither the use of III-V materials for the optically active aspect nor the integration and/or use thereof with silicon seem to be disclosed in this document. Operation at <1200 nm seems to be described which may make the disclosed device unattractive for light propagation using silicon waveguides, for example. Also, this document does not seem to provide instruction on how to address the problems/issues associated with hybrid/heterogeneous integrated optical systems as hereinbefore described.
US 2008/0002929A1 describes an apparatus and a method for electrically pumping a hybrid evanescent laser. For one example, the apparatus includes an optical waveguide disposed in silicon. An active semiconductor material is disposed over the optical waveguide defining an evanescent coupling interface between the optical waveguide and the active semiconductor material such that an optical mode to be guided by the optical waveguide overlaps both the optical waveguide and the active semiconductor material. A current injection path is defined through the active semiconductor material and at least partially overlapping the optical mode such that light is generated in response to electrical pumping of the active semiconductor material in response to current injection along the current injection path at least partially overlapping the optical mode. In this document, the light generated by the active semiconductor material is evanescently coupled to a silicon waveguide that constitutes a passive aspect. The presented approach for facilitating a light source on silicon is based on hybrid/heterogeneous integration rather than directly by monolithic integration. Because the active semiconductor material is remotely positioned with respect to the silicon waveguide, it may be that the position of the generated light relative to passive aspect is relatively unchanged. Also, it may be that the overlap of the generated light with the active semiconductor material is relatively small, which is concurrent with a hybrid mode of operation, that is, a mainly passive mode with a relatively smaller active mode. Such a hybrid mode of operation may cause relatively higher threshold currents and lower optical output levels.
US 2008/0198888 A1 discloses a method of bonding a compound semiconductor on a silicon waveguide for attaining a laser above a silicon substrate. This document is concerned with the heterogeneous integration, rather than the monolithic integration, of a light source based on a compound semiconductor material system with respect to a silicon substrate.
US2009/0245298A1 discloses a silicon laser intermixed integrated device, comprising: a silicon-on-insulator substrate comprising at least one waveguide in a top surface, and a compound semiconductor substrate comprising a gain layer, the compound semiconductor substrate being subjected to a quantum well intermixing process, wherein the upper surface of the compound semiconductor substrate is bonded to the top surface of the silicon-on-insulator substrate. This document is concerned with the hybrid/heterogeneous integration, rather than the monolithic integration, of a surface of a compound semiconductor substrate with respect to a silicon-on-insulator substrate. Based on the index contrasts of the fabricated structures, it may be that the light generated by the laser source/compound semiconductor aspect is mainly confined in the silicon with a relatively small proportion being confined within the compound semiconductor, which may serve to limit the efficiency of the laser and result in relatively increased power consumption.
U.S. Pat. No. 5,703,896 discloses an apparatus for emitting varying colours of light comprising: a lasing layer formed of crystalline silicon quantum dots formed in an isolation matrix of hydrogenated silicon; said quantum dots being formed in three patches; each of said three patches having different sized quantum dots therein to thereby produce three different colours of light; a barrier layer of p-type semiconductor under said lasing layer, said p-type semiconductor being selected from the group GaP, SiC, GaN, ZnS; a substrate member under said barrier layer; an n-type semiconductor layer above said lasing layer, said n-type semiconductor layer being selected from the group GaP, SiC, GaN, ZnS; a positive potential contact beneath said substrate member, three negative potential contacts; each of said three contacts being above a different one of said three patches; each of said three contacts acting with said positive contact to selectively bias a different one of said three patches; three sectors of concentric grating surrounding said three patches; each of said sectors having a radial period corresponding to the colour of light produced by an adjacent one of said three patches; and each of said sectors resonating photons emitted by said adjacent patch to stimulate coherent light emission. This document is concerned with the fabrication of silicon quantum dots in silicon. It does not address the monolithic integration of a light/laser source based on a compound semiconductor such as, a III-V material system, with respect to an optical structure such as, a photonic structure and/or optical waveguide based on a silicon platform.
US2007/0105251 discloses a laser structure comprising: at least one active layer including doped Ge so as to produce light emissions at approximately 1550 nm from the direct band-gap of Ge; a first confinement structure being positioned on a top region of said at least one active layer; and a second confinement structure being positioned on a bottom region of said at least one active layer. This document describes the fabrication and the use of a laser on a silicon platform. The gain medium seems to be based on trained and doped germanium, which mimics a direct band-gap material, rather than a compound semiconductor material system such as a III-V material system, for example. Certain cavity designs, such as vertical cavity self-emitting lasers (VCSELs) are contemplated in the discussed disclosure. This document does not seem to provide instruction on how to address the problems/issues associated with hybrid/heterogeneous integrated optical systems as hereinbefore described.
US2007/0104441 discloses an integrated photodetector apparatus comprising: (a) a substrate comprising a first cladding layer disposed over a base layer, the base layer comprising a first semiconducting material, the first cladding layer defining an opening extending to the base layer; (b) an optical waveguide comprising the first semiconductor material and disposed over the substrate; and (c) a photodetector comprising a second semiconductor material epitaxially grown over the base layer at least in the opening, the photodetector comprising an intrinsic region optically coupled to the waveguide, at least a portion of the intrinsic region extending above the first cladding layer and laterally aligned with the waveguide. The disclosed fabrication method is in relation to a germanium photodetector that is laterally coupled to a polycrystalline waveguide and is aligned relative thereto by way of a dedicated, multiple-step alignment procedure. This document does not address how a compound semiconductor based light source may be monolithically integrated with respect to integrated optics based on a silicon platform.
US2010/0295083A1 discloses a multilayer structure containing a silicon layer that contains at least one waveguide, an insulating layer and a layer that is lattice compatible with Group III-V compounds, with the lattice compatible layer in contact with one face of the insulating layer, and the face of the insulating layer opposite the lattice compatible layer is in contact with the silicon layer. The silicon and the insulating layers contain either or both of at least one continuous cavity filled with materials such as to constitute the photodetector zone, or at least one continuous cavity filled with materials such as to constitute a light source zone. A multilayer wafer structure is disclosed in this document comprising in descending order: a surface silicon layer, a silicon dioxide layer, a lattice compatible layer, a buried oxide layer and a silicon base. The surface silicon core layer is structured to comprise a waveguide structure and the silicon dioxide layer comprises a cladding layer. The surface silicon layer and the silicon dioxide layer are structured to comprise a cavity that extends to an upper surface of the lattice compatible layer. The cavity is filled with materials thereby to facilitate an optically active aspect comprising a light source zone and/or a photodetector zone. The lattice compatible layer acts as a template for growth of the light source zone and the photodetector zone and it most preferably comprises germanium. With respect to the lattice compatible layer comprising germanium and as acknowledged in this document: the growth of high-quality epitaxial germanium on silicon is non-trivial due to the misfit dislocations that form due to the lattice mismatch, and that subsequent/any processing steps should be conducted so as to avoid inducing damage in the germanium surface or removing such damage afterwards, since the homoepitaxy/heteroepitaxy on germanium requires a good crystalline template. Furthermore, the lattice compatible layer has to be bonded to the upper silicon dioxide layer, which may increase the number of processing steps and/or complexity in the fabrication of the device disclosed in this document. Also, this document provides the instruction that: incorporation of any electronic circuits and waveguides in the silicon layer is done prior to the preparation of the light source and the photodetector zones since optically active materials such as III-V compounds risk contaminating and damaging the silicon device fabrication process, and that the processing steps for the incorporation of such electronic circuits and/or the waveguiding structures should not exceed the melting point of the material that is used for the lattice compatible layer.
Accordingly, it is a challenge to provide a semiconductor device for use in any given optical application that provides an improved performance, particularly in respect of lower threshold currents and/or higher optical output power, over previously-proposed optical systems, for example, those based on heterogeneous/hybrid integrated optical systems as above-discussed. It is also a challenge that, in respect of such a semiconductor device, an optically active aspect based on a compound material system, such as, for example, a III-V material system, is directly and monolithically integrated on an integrated optical platform based on silicon, for example.